The invention relates to a spindle motor, in particular for driving the storage disk(s) of a hard disk drive, with a stator, a rotor, and a shaft disposed between stator and rotor, which bears at least one roller bearing, which comprises an inner and an outer bearing ring with roller bodies disposed between them.
For reasons of the accuracy of running such a spindle motor, conventionally prestressed roller bearing pairs are used, with the aid of which the rotor is supported rotatably relative to a stator disposed in a base plate. Therein the prestress impressed upon the bearing system causes elastic deformations at each of the contact sites of the roller bodies with the bearing rings. The degree of deformation is a function, for one, of the bearing geometry, thus of the radii of the inner and outer raceway, of the diameter of the roller bodies and of the radial play, for another, of the amplitude of the xe2x80x9cfrozen-inxe2x80x9d prestress force. In order for the read/write function of a hard disk drive to be ensured, (the read/write head mounted on a pivotable arm moves at a flying height of only 0.001 to 0.002 mm relative to the disk surface), apart from the running accuracy of the motor, the surface of the rotating storage disk must extend at every site and at every point in time as much as possible at right angles with respect to the axis of rotation. This means that the axial or front wobble of the disk must be very low. In order to attain minimum values in this regard, a pressfit is conventionally used for the seat of the roller bearing rings in the corresponding bore of the rotor. The interference fit, accordingly constructively specified, of the bearing ring(s) in the fit joint permits to develop in the joined parts tangential and radial stresses which are greater the greater the interference fit of the joined parts.
These stresses bring about, on the one hand, a widening of the bore in the rotor and, on the other hand, a constriction of the roller bearing outer ring(s) and thus a decrease of the xe2x80x9cnominalxe2x80x9d radial play in the roller bearing(s) accordingly in consequence of the growth of the degree of the elastic deformations.
But since the elastic deformations are a measure of the rigidity of the entire system, the oscillatory behavior of this system also changes with the degree of elastic deformations. At the same inertia, thus, at a high degree of elastic deformation, the intrinsic frequency of the resonant xe2x80x9cspring-mass systemxe2x80x9d is high and accordingly low at a low increment of elastic deformation. A further component determining the intrinsic frequency of the rotating system are the storage disks fastened on the rotor whichxe2x80x94viewed by themselvesxe2x80x94also represent a resonant system with system-specific intrinsic frequency.
Thus, overall a total resonant system is to be considered which comprises essentially two spring-mass systems coupled to one another, whose system-specific parameters determine the intrinsic resonance frequency(-ies) of the total system impairing function.
Important for the function of the hard disk drive is that the system-immanent dynamic disturbances, caused in particular by form deviations of the roller bearing components, do not coincide with the intrinsic frequencies of the total system. This requirement must be ensured under all operating conditions since, otherwise, the oscillation amplitudes generated at resonance can readily lead to read/write errors. Due to the, to some extent, rather small critical safety spacing, further narrowed between intrinsic and interference frequencies, the intrinsic frequencies of the total system should remain as much as possible constant in the relevant temperature range of approximately +5xc2x0 C. to +55xc2x0 C. If this is not ensured, especially with the enormously increasing storage densities of current hard disk drives, fabrication inaccuracies of the roller bearing can lead to an undesirable excitation of the intrinsic frequency(-cies) and thus to non-acceptable function disturbances in the form of extremely long access times up to the processing of defective or wrong data.
Within the current state of the art aluminum is customarily used for the rotor and roller bearing steel [is used] for the bearing. The coefficient of thermal expansion of these two materials, however, differs by the factor 2 since xcex1-aluminum is approximately 24*10xe2x88x926[Kxe2x88x921] and xcex1-steel approximately 12*10xe2x88x926[Kxe2x88x921]. This means that with increasing temperature the aluminum of the rotor enclosing the roller bearing outer rings expands more strongly than the bearing rings themselves. Thereby, with increasing temperature, the interference fit decreases in the fit joint whereby the xe2x80x9cnominalxe2x80x9d radial play in the bearing increases. But greater radial play means a decrease of the elastic deformation whereby the rigidity of the total system decreases. Since the modulus of elasticity of the storage disk also decreases with increasing temperature, an undesirable temperature dependence of the intrinsic frequency of the total system results. Thus, the intrinsic frequency, contrary to the initially posed requirement, decreases with increasing operating temperature or, conversely, increases with decreasing temperature which, especially in the case of fixed disks with high storage density, can lead to function disturbances during operation.
It is therefore the task of the invention to counteract a temperature-dependent change of the radial play of the roller bearings as a cause of the temperature dependence of the intrinsic frequency, thus to minimize or eliminate it completely.
This task is solved according to the invention by the characteristics of the independent patent claims.
In a first embodiment the invention is distinguished thereby that at least one equilibration element is present acting upon an outer and/or inner bearing ring, which is disposed between the corresponding bearing ring and a part bearing the bearing ring, and which has a specified coefficient of thermal expansion xcex1. The equilibration element is preferably structured annularly, like the bearing rings themselves.
The equilibration element is preferably disposed directly on the stator, on the shaft or on the rotor, with the equilibration element being capable of bracing itself on suitable surfaces of these structural components.
A development of the invention provides that the outer bearing ring is for the most part encompassed by the equilibration element. The equilibration element preferably comprises a material with low coefficient of thermal expansion, such as, for example ceramic material. Since the equilibration element expands only slightly at an increase of the temperature, the outer ring is forced to become shifted inwardly in the direction of the inner ring in the sense of a constriction. The decrease of the nominal radial play caused thereby counteracts a temperature-dependent decrease of the bearing prestress.
In another development of the invention an equilibration element is used which comprises a material having a high coefficient of thermal expansion xcex1, preferably xcex1 greater than 12*10xe2x88x926[Kxe2x88x921].
The equilibration element can be disposed such that it expands in the axial direction at a temperature increase and exerts a force on the associated bearing ring in the axial direction. Likewise, the equilibration element can be disposed such that at a temperature increase it expands in the radial direction and exerts a force on the associated bearing ring in the radial direction. A combination of axial and radial action of one or several equilibration elements on one or several bearing rings is also possible.
A further development of the invention provides that the equilibration element is not disposed directly on the shaft but on an annular insert disposed on the shaft. This allows for the simpler positioning of the equilibration element, in particular if the equilibration element is to act in the axial direction onto a bearing ring.
The annular insert preferably comprises a material with a low coefficient of expansion xcex1, which means, at a temperature increase, a reference surface expanding only slightly for the equilibration element is formed. But, instead of a separate annular insert, a ring connected integrally with the shaft or a collar formed onto the shaft can also be provided.
According to the invention it is insignificant whether the equilibration element comprises one or several parts.
As materials with high coefficients of expansion xcex1 for the production of the equilibration element aluminum, aluminum alloys or synthetic material can be used. As materials with low coefficient of expansion xcex1 steel, steel alloys or ceramic materials are possible.
In a further embodiment of the invention the shaft itself is used as the equilibration element. In this case, the shaft comprises a material with low coefficient of expansion xcex1. The inner rings are disposed on the shaft and fixedly connected with it. The coefficient of thermal expansion of the shaft is preferably lower than the coefficient of thermal expansion of outer and inner ring(s).
As an alternative to the use of separate equilibration elements for the compensation of the xe2x80x9cnominalxe2x80x9d radial play with temperature variations, the roller bodies themselves can be used as equilibration elements. Herein the roller bodies comprises a material with a higher coefficient of thermal expansion xcex1, preferably xcex1 greater than 12*10xe2x88x926[Kxe2x88x921], while the associated outer and/or inner bearing rings comprise a material with a lower coefficient of thermal expansion xcex1, preferably a  less than 12*10xe2x88x926[Kxe2x88x921].
In order to be able to control the effect of the expansion of the roller bodies better, it is preferred if the coefficient of expansion of the roller bodies is substantially higher than the coefficient of expansion of the bearing rings or of the remaining bearing components.